As is well known, in an FM (frequency modultion) stereo receiver, a composite stereo signal which includes a pilot signal of 19 Hz, a main signal indicative of the sum of the left and right channel signals, and a sub signal indicative of the difference between the left and right channel signals, where the sub carrier of 38 Kz is suppressed, is first detected and then the detected composite stereo signal is fed to a stereo multiplex demodulator or decoder to extract the left and right channel signals from the composite stereo signal respectively. The pilot signal of 19 Hz is essential for obtaining a difference signal (L-R) and an inverted difference signal (R-L) by demodulating the sub signal. However, the pilot signal is unnecessary after the left and right channel signals are obtained. Namely, if the detected left and right channel signals include pilot signal components, i.e. the pilot signal and its harmonics, these pilot signal components have to be eliminated from the demodulated left and right channel signals by means of a suitable low pass filter. Provision of such a low pass filter may deteriorate the frequency response characteristic of the demodulated signals.
For the above reason, in a conventional stereo multiplex demodulator, a pilot signal cancelling signal is first produced and the pilot signal components included in the composite stereo signal is negated or cancelled by the cancelling signal during demodulation. As will be described in detail hereinlater, when a pilot signal cancelling signal is used to eliminate the pilot signal components in a conventional manner, the pilot signal components included in the demodulated left and right channel signals cannot be sufficiently eliminated to a practical level. Therefore, in a conventional stereo multiplex demodulating system, a low pass filter has to be employed to further reduce the pilot signal components. When such a low pass filter is used, the flatness of the frequency characteristics as to the high frequency components of the demodulated signals is apt to be deteriorated. For this reason, the degree of the pilot signal components elimination or elimination ratio cannot be set large enough. In order to solve this problem, it is theoretically possible to employ a resonance circuit consisting of a coil and a capacitor for producing a pilot signal cancelling signal which hardly includes harmonic components thereby obtaining high degree of elimination of the pilot signal components. However, this countermeasure has drawbacks as follows: Namely, the coil included in the resonance circuit is apt to induce inductive noises, while the phase of the cancelling signal tends to vary due to the variations of the constants of the coil and the capacitor. As the phase of the cancelling signal deviates from a given point, the degree of elimination drops resulting in the unstability of the pilot signal components elimination.
Furthermore, a coil occupies a relatively large space so that the stereo multiplex demodulator including such a resonance circuit tends to be bulky although remaining elements can be included in an integrated circuit. Consequently, the above mentioned theoretically possible measure cannot be adopted.